The present invention relates generally to a method for producing an arsenic doping in silicon semiconductor substrates. More specifically, the present invention relates to a method for producing a defined arsenic doping in the side walls and floors of trenches having a high aspect ratio which are etched into a silicon semiconductor substrate, whereby an arseno-silicate glass layer deposited into these trenches is used as a diffusion source, this layer being removed after the diffusion.
In the development of VLSI circuits, for example the 4 megabit dynamic RAM, the planar cells are being increasingly replaced in the capacitor memory cells by trench (well) cells. This is due to the planar cells' greater space requirements. In order to increase the packing density of the memory cells, and thereby retain the storage capacity, trenches having a high aspect ratio of greater than or equal to 3 (the ratio of the depth of the trench to the width) are generated. In order to avoid leakage currents to neighboring memory cells and guarantee a high reliability of the circuit, a precisely controlled, steep n.sup.+ profile having low penetration depth is also required.
Given the high aspect ratio of such trenches, the drive-in from the doped glass layers (diffusion source) deposited there has been found to work satisfactorily for doping the side walls and floors. A more uniform doping is thereby achieved and the decrease of the dopant concentration in the direction toward the trench floor is reduced.
Arsenic is preferred as a dopant for generating conductivity regions of the n.sup.+ type. The diffusion length thereof is lower by a factor of 4 in comparison to phosphorous and thereby enables smaller spacings of the trenches.
A method for producing an arsenic doping in drive-in from doped glass layers is disclosed in an article by K. Yamada et al, IEDM Technical Digest 1985, pages 702-708. The method disclosed utilizes a LPCVD (low pressure chemical vapor deposition) reactor. An arseno-silicate glass layer (AsSG) is generated by the thermal decomposition of triethylarsenite (As(OC.sub.2 H.sub.5).sub.3, TEAsite) and tetraethylortho-silicate (TEOS, Si(OC.sub.2 H.sub.5).sub.4) and arsenic is driven into the substrate by a high-temperature process.
The disadvantages of this method include the fact that there is a high material consumption relative to the obtainable level of the doping. In view of the high material costs involved, this makes the method not entirely satisfactory from an economic standpoint. Moreover, the decomposition products that result from the process make frequent cleaning of the filters, pumps, and other vacuum parts necessary. This creates further problems due to the toxicity of these decomposition products that include arsenic-containing components. As a consequence of the frequently required cleanings, a low throughput of the LPCVD system per time unit is obtained, this likewise makes the process more costly and less economical.
Further, in order to achieve a steep doping profile, it is necessary to pre-treat the substrate. Furthermore, it is necessary to set an exact atmosphere composition during the high-temperature diffusion process (drive-in).
There is therefore a need for an improved method for producing a defined arsenic doping.